1. Field of the Invention
This invention relates to apparatus for removing ionic impurities from electroplating baths and similar electrolyte solutions, and in particular to a special electrolytic cell that is adapted to remove these impurities when immersed in the electrolyte solution being purified.
2. Prior Art
Electrodialysis is a process that involves the transport of ions through a ion permeable membrane as a result of an electrical driving force, and the process is commonly employed to remove unwanted ions or ionic impurities from aqueous electrolyte solutions -- as, for example, in the desalinization of brines and the like. The process is commonly carried out in an electrodialysis cell having an anolyte compartment and a catholyte compartment separated by a permselective membrane, the anolyte compartment and the catholyte compartment each containing an electrolyte solution. If the ionic impurities are anions (for example, mineral acid ions), the permselective membrane is an anion permeable material and the solution to be purified is placed in the catholyte compartment of the cell. The passage of an electric current between the anode and the cathode of the cell causes the contaminant anions present in the catholyte solution to migrate from the catholyte through the anion permeable membrane into the electrolyte solution (the anolyte) contained in the anolyte compartment, the reverse migration of cations from the anolyte solution into the catholyte solution being largely prevented by the anion permeable membrane. Similarly, if the ionic impurities are cations (for example, metal ions), the permselective membrane is a cation permeable material and the solution to be purified is placed in the anolyte compartment of the cell. The passage of an electric current between the anode and the cathode of the cell causes the contaminant cations present in the anolyte solution to migrate from the anolyte through the cation permeable membrane into the electrolyte solution (the catholyte) contained in the catholyte compartment, reverse migration of anions from the catholyte solution into the anolyte solution being largely prevented by the cation permeable membrane.
By way of example, it has heretofore been proposed in U.S. Pat. No. 3,909,381 to Robert F. Ehrsam that certain harmful metal impurities (principally, iron and copper ions) be removed from chromium plating solutions by electrodialysis of the solution. In this process the chromium plating solution to be purified comprises the anolyte solution and an ionizable organic compound having the characteristics described in the patent comprises the catholyte solution, the anolyte and catholyte solutions being separated from each other by a cation permeable membrane. When an electrolyzing current is passed between the anode and the cathode of the cell, the unwanted ferric and cupric ions migrate from the chromium plating solution contained in the anolyte compartment through the cation permeable membrane into the catholyte solution contained in the catholyte compartment, reverse migration of anions from the catholyte solution into the anolyte solution being largely prevented by the cation permeable membrane.
The electrodialysis process described in the Ehrsam patent removes cationic impurities from chromium plating solutions efficiently and economically. However, it has been found that the ferric and cupric anions that are caused to migrate into the catholyte solution are there reduced to lower valent forms that tend to precipitate from the catholyte solution. Chelating agents can be added to the catholyte solution to substantially prevent the precipitation of iron and, to a lesser extent, inhibit the precipitation of copper from the solution. However, it is difficult to prevent entirely the precipitation of metallic copper, and finely divided particles of this metal tend to be electrodeposited on the surface of the cathode and to be precipitated from the catholyte solution in the immediate vicinity of the cathode where they can have a serious disruptive effect on the operation of the electrodialysis cell. Moreover, it has been found that the process is difficult to carry out in conventional electrodialysis cells due to the corrosive nature of the anolyte solution and the need for frequent interruption of the process for disassembly and cleaning of the catholyte compartment of the cell.
After an intensive investigation of the problems caused by the precipitation of deleterious materials on and in the immediate vicinity of the electrodes of an electrodialysis cell and the consequent need to interrupt the electrodialysis process for disassembly and cleaning of the cell, I have devised a novel electrolytic cell for removing ionic impurities from electrolyte solutions by electrodialysis of the solution, the cell being provided with means for mechanically removing precipitated particulate material from the anode or the cathode of the cell, as hereinafter described, without the need for frequent disassembly of the cell to remove the precipitated material. The electrodialysis cell I have devised is efficient and reliable in operation and is applicable to the purification of all types of electrolytic solutions wherein the aforementioned problems exist.